Plants growing in containers require frequent watering at varying time intervals, the duration of which depends on the rate of absorption and evaporation. Special containers are commercially available which provide moisture for a prolonged time and reduce the need for frequent watering. This type of containers are comprised of two compartments, one contains a growth medium that supports the plant and the other contains water or a water solution of fertilizer. Solution from the latter compartment is drawn into the growth medium of the first compartment by the capillary action of a wick which is made in contact with both the solution and the growth medium. Alternatively, in a similar type the two compartments are constructed to share a common wall, part or all of this common wall is made of a porous material that allows a slow flow of solution from the compartment that contains it into the compartment sustaining the plant.
In the continued discussion references are made to the following sources and U.S. patents:
SUNSET magazine (vol.182) page 183, February 1989. PA1 U.S. Pat. No. 2,031,146: Dodge PA1 U.S. Pat. No. 2,611,643: Higgins PA1 U.S. Pat. No. 3,293,799: Keller et al. PA1 U.S. Pat. No. 3,438,575: Rohling PA1 U.S. Pat. No. 3,512,712: Benesch PA1 U.S. Pat. No. 3,892,982: Holmes PA1 U.S. Pat. No. 4,937,972: Freitus
An article that appeared in Sunset magazine points out that double compartment containers of the type described above have major drawbacks. One of those is the lack of control of the amount of moisture around the root ball of the plant in the growth compartment: the root ball becomes saturated with water and damage to the plant results. Another drawback is the growth of roots close to, and into the wick or the porous wall. As a result the flow of water to the plant is gradually reduced, requiring periodical removal of the plant for root trimming. The article also points out the availability of an improved variation on the dual compartment systems: an additional moisture sensor. The same type of sensor is also described by Rohling. In this type of system the compartment that contains the solution is made fully enclosed. Inflow of air to the portion of it above the solution level is allowed through a flexible tube. This tube is connected on its opposite end to a moisture sensor which is inserted into the growth medium. As long as the sensor is moist it shuts off the flow of air into the solution compartment thereby shutting off the flow of solution into the growth compartment. When the moisture level in the growth medium falls, the sensor drys up and allows air to enter the compartment which contains the solution and in turn allows solution to flow into the growth compartment. The solution then spreads throughout the growth medium eventually reaches and moistens the sensor which shuts off the flow. Positioning the sensor at different locations within the growth medium relative to the location at which solution enters the growth compartment allows control of the amount of solution delivered to the growth medium.
Freitus describes a self contained plant growth system comprised of separate compartments for the growth medium and the feeding solution. Solution is pumped out of the solution compartment by an electrically powered pump and delivered to the growth medium. A moisture detector is inserted in the growth medium and communicates with the pump electrical circuit. When the moisture level in the growth medium falls below a preset level the sensor sends a signal to activate the pump. Consequently, solution enters the growth medium via a supply tube that extends from the pump towards a location above the growth medium. Solution falls freely from the opening of the tube down to the growth medium below at some distance from the location where the sensor is inserted. The solution is being absorbed by the growth medium. The resulted increase in moisture is then detected by the moisture sensor which in turn sends a signal to stop the pump. Adjusting the amount of solution delivered to the growth compartment in the Rohling and in the Freitus systems is difficult because it involves a trial and error effort to establish the proper location of the sensor.
For the automatic feeding of plants that are planted in common containers, i.e. containers with a single compartment that supports the plant and its growth medium, add-on provisions were devised. One provision is to supply water from the regular water supply line via a timer operated valve. Alternatively, the valve can be connected to an elevated container which contains water or fertilizer solution thus making use of the gravitational force. The timer is programmed to open the valve periodically and for a preset duration. The selection of proper timer settings for optimal feeding is difficult. With this method over-feeding or under-feeding are likely and damage to the plant results. Provisions which make use of a constant flow of water are also in use: plant containers are placed on an absorptive mat which is kept soaked by the constant feed of water. Excess of water is allowed to drain off. Water enters the containers via holes on the bottom. In this method over-feeding and a resulting damage to plants is very likely. As was made clear in the prior discussion, solution flow controlled by a moisture sensing device is essential for reliable operation.
Keller et al. suggest a system that exploit the difference is gross weight of the container and its contents between the properly moistened condition and the dryer, yet sufficiently moist condition, for determining moisture level. The plant container is placed on a platform which is a part of a weight sensing mechanism. A valve which is mechanically controlled by this mechanism opens the supply of solution to the container when its weight drops below a presettable level and shuts it off when the container weight reaches a higher presettable level. The main drawback of this method stems from the fact that most plants gain in weight at a significant rate. Consequently, the amount of feed is gradually reduced and proper moisture cannot be maintained. Another drawback of these systems is their limited ability to accommodate containers of arbitrary size and weight. Also their complexity and cost precludes them from wide use.
Benesch suggests a system which include a moisture sensor for flow control that exploits the property of wood to expand when moist and contract when dry. This expansion and contraction is used to activate a valve that controls the flow of water or feeding solution to the growth medium. This system shares the same disadvantage as those suggested by Rohling and by Freitus because the amount of solution delivered depends on the time it takes for the moisture to spread and wet the sensor element thus making it difficult to set up.
Dodge, Higgins and Holmes separately suggest systems that exploit the change in the electrical resistance between a set of two conductive electrodes embedded in the growth medium for sensing the moisture level of that medium. Since determination of the resistance requires flow of electric current through the growth medium, the current must be low enough as to prevent excessive chemical decomposition within the growth medium due to electrolysis. And since the changes of this current are used to control an electrically actuated flow valve, current amplification is required. Dodge uses electromechanical relays as amplifiers. Higgins and Holmes use electronic valves in combination with electromechanical relays. In addition, Holmes added a controllable timer which in response to a signal from the moisture sensing circuit opens the flow of feeding solution to the growth medium and shuts it off after a preselected time. This facilitates the prevention of over-feeding.
The main objective of the invention is to provide an inexpensive add-on provision for the automatic feeding of potted plants that are normally being fed manually. In particular this invention will be of help to those who plan to be absent from their home or office for many months and wish to provide continuous and cost effective care for their house or office plants during their absence.
In light of the prior discussion of prior art the following specific goals of the invention should now be clear:
a. To deliver water or plant nutrient solution out of an arbitrary container selected and provided by the user. PA0 b. To initiate feeding only when the moisture level in the growth medium falls bellow a predetermined level. PA0 c. To deliver a controllable amount of plant nutrient solution at each feeding cycle. PA0 d. To be self powered and portable. PA0 e. To be of a ready to use integral construction thus avoiding the need by the user to assemble mechanical parts or to connect electric wires. PA0 f. To provide a cost effective feeding for even the least expensive potted plant.
The above objectives are achieved by constructing an apparatus consisting of a motor-driven pump, a flexible tube for the delivery of solution from the pump to the plant container, a stake assembly to facilitate anchoring of the tube and a set of two moisture sensing electrodes to the growth medium, and electric battery which supplies power to the motor via an electronic switching circuit which responds to the signal from the moisture sensing electrodes. The user selects a container, fills it with water or nutrient solution and places it next to the potted plant. Then he inserts the stake assembly into the growth medium and inserts the pump into the container. The operation of the apparatus starts automatically thereafter.